并发编程(2)-多个线程多个锁

清疚 2022-06-06 08:54 195阅读 0赞

对象锁和类锁

package com.thread.mythread.conn002;
    
    public class MultiThread {
    
        private int num = 0;
    
        public synchronized void printNum(String tag){
            try {
                if("a".equals(tag)){
                    num = 100;
                    System.out.println("tag a, set num over");
                    Thread.sleep(1000);
                } else {
                    num = 200;
                    System.out.println("tag b, set num over");
                }
                System.out.println("tag "+tag+",num "+num);
            } catch (Exception e) {
                // TODO: handle exception
            }
        }
    
        public static void main(String[] args) {
    
            final MultiThread multiThread1 = new MultiThread();
            final MultiThread multiThread2 = new MultiThread();
            Thread t1 = new Thread(new Runnable() {
                public void run() {
                    multiThread1.printNum("a");
                }
            });
            Thread t2 = new Thread(new Runnable() {
                public void run() {
                    multiThread2.printNum("b");
                }
            });
            t1.start();
            t2.start();
        }
    }

运行结果：

> tag a, set num over  
> tag b, set num over  
> tag b,num 200  
> tag a,num 100

按照上一篇介绍，对printNum方法添加了synchronzied关键字，应该是一个线程完成，才会执行其他的线程，那么结果应该就是

> tag a, set num over  
> tag a,num 100  
> tag b, set num over  
> tag b,num 200

为什么这里的结果不是预期的结果呢？其实这里使用了两个对象，两个锁。对象t1和对象t2是两个独立的对象，两把不同的对象锁。如果把printNum方法上添加static关键字，那么printNum就属于对象级的方法，无论是多个对象t1，t2，tn，获取的都是类级别的锁。

package com.thread.mythread.conn002;
    
    public class MultiThread {
    
        private static int num = 0;
    
        public static synchronized void printNum(String tag){
            try {
                if("a".equals(tag)){
                    num = 100;
                    System.out.println("tag a, set num over");
                    Thread.sleep(1000);
                } else {
                    num = 200;
                    System.out.println("tag b, set num over");
                }
                System.out.println("tag "+tag+",num "+num);
            } catch (Exception e) {
                // TODO: handle exception
            }
        }
    
        public static void main(String[] args) {
    
            final MultiThread multiThread1 = new MultiThread();
            final MultiThread multiThread2 = new MultiThread();
            Thread t1 = new Thread(new Runnable() {
                public void run() {
                    multiThread1.printNum("a");
                }
            });
            Thread t2 = new Thread(new Runnable() {
                public void run() {
                    multiThread2.printNum("b");
                }
            });
            t1.start();
            t2.start();
        }
    }

执行结果：

> tag a, set num over  
> tag a,num 100  
> tag b, set num over  
> tag b,num 200